Conventionally, pneumatic-operated ON/OFF valves used for high-pressure gas cylinders have been satisfactorily achieved in terms of automatization of the valve-opening/closing operation but have not satisfactorily complied with the demand for a slowing-down of the valve opening speed. For instance, when certain types of gases are mixed in a reaction chamber, it is sometimes required that the gasses to be mixed are supplied into the reaction chamber as slow as possible. Applicants including the applicant of the present invention and inventors including the inventor of the present invention have proposed a slow-acting ON/OFF valve capable of satisfying such a demand and have obtained patents for such a valve. See Japanese Patent Publication No. 2,784,446 and Japanese Patent Publication No. 3,359,953. In addition, a different solution for the same problem has been proposed by not only these applicants including the applicant of the present invention but also by other applicants. See Japanese Patent Application Publication No. H11-30399.